1. Field of the Invention
The present invention relates to an image forming apparatus which reads a document image and transfers it onto a sheet of paper.
2. Description of the Related Art
For image forming apparatuses such as photocopiers, the surface of a photosensitive drum, which is an image bearer, is scanned with a laser beam to form an electrostatic latent image on the surface of the photosensitive drum. The electrostatic latent image is developed (visualized) by a developing agent and transferred to a sheet of paper.
The laser beam emitted from a laser diode hits a galvano-mirror, and is reflected by the mirror to hit a polygon mirror, where it is reflected again. With rotation of the polygon mirror, the surface of the photosensitive drum is linearly scanned along an axial direction of the photosensitive drum. This line scanning is referred to as main scanning. Main scanning is repeatedly performed with the rotation of the photosensitive drum. The main scanning successively shifted with the rotation of the photosensitive drum is referred to as sub-scanning.
The galvano-mirror reflects the laser beam for the scanning, and rotates by an amount corresponding to a level of inputted driving voltage so that a reflection angle changes. By the rotation of the galvano-mirror, a scanning position of the laser beam can be adjusted in a sub-scanning direction.
In an example shown in Jpn. Pat. Appln. KOKAI Publication No. 2001-91872, the scanning position of the laser beam in the sub-scanning direction is optically detected, and the galvano-mirror is driven/controlled in such a manner that the detected scanning position becomes appropriate.
To drive the galvano-mirror, a digital/analog conversion unit (hereinafter referred to as a D/A conversion unit) is disposed into which digital data for continuously and finely adjusting the reflection angle of the galvano-mirror is inputted. When the digital data is inputted into the D/A conversion unit, the voltage of a level corresponding to the digital data is outputted from the D/A conversion unit. This output voltage is a driving voltage of the galvano-mirror.
Concretely, an input value of the digital data is controlled to increase/decrease (feedback-controlled) with respect to the D/A conversion unit so as to allow the galvano-mirror to rotate in a direction in which a scanning position of the laser beam is in a predetermined position.
To adjust the scanning position of the laser beam at a high resolution without narrowing the adjustment range in driving/controlling the galvano-mirror as described above, for example, a D/A conversion unit with a high resolution of a 16-bits type is used.
Additionally, the D/A conversion unit with a high resolution of the 16-bits type is expensive, and there is a problem that a cost rise of the apparatus results.